1. Field of the Invention
The present invention relates to a rotary machine and an electromagnetic clutch suitable for use in a compressor of an automotive air conditioning system, and a belt transmission adjusting the driving belt tension of a vehicle.
2. Description of Related Art
FIG. 4 shows a conventional electromagnetic clutch 100 used for a compressor of an automotive air conditioner. The electromagnetic clutch 100 includes a pulley 101, a rotor 102, an electromagnetic coil 103, an armature 104 and a hub 105. The pulley 101 and the rotor 102 rotates by receiving a rotational force from a vehicle engine through a belt 110, and the armature 104 is placed to face the rotor 102 with a predetermined slight gap therebetween. The armature 104 is connected to a compressor shaft 121 through the hub 105. When the electromagnetic coil 103 is energized, the armature 104 couples to the rotor 102, and the rotation of the rotor 102 is transmitted to the compressor shaft 121 through the armature 104 and the hub 105.
A bearing 106 is provided between the inner periphery of the rotor 102 and a cylindrical boss portion 122 of the compressor 120. The bearing 106 rotatably supports the rotor 102.
In general, the bearing 106, used in the electromagnetic clutch of the compressor, is a double row angular contact ball bearing. Such a bearing 106 includes an inner race 106a, an outer race 106b, and a plurality of balls 106c arranged in double rows between the inner and outer races 106a and 106b, and has a contact angle .alpha.. Each ball 106c contacts both inner and outer races 106a, 106b at one point respectively. Here, in FIG. 5, numerals 201-208 denote the contact points between the balls 106c and the inner and outer races 106a, 106b.
However, in the above described electromagnetic clutch, the bearing 106 creates unwanted noise. This bearing noise especially occurs when the rotation speed of the compressor 120 is low, the temperature around the compressor 120 is low, grease in the bearing 106 leaks, or the viscosity of the grease is low.
As shown in FIG. 4, an action line E of a tension D of the belt 110 axially diverts from the radial central line F of the bearing 106. In the bearing 106, slight gaps (about 40.mu.) are provided between the balls 106c and the inner and outer races 106a, 106b in the radial direction. Thus, a moment caused by the tension D of the belt 110 makes the outer race 106b incline in the counter clockwise direction. Similarly, the pulley 101 and the rotor 102 are made to incline.
Here, as the balls 106c are arranged in double row, even when the outer race 106b inclines, the top ball 106c in the left row in FIG. 5 contacts both inner and outer races 106a, 106b at the contact points 201, 202. Similarly, the bottom ball 106c in the right row contacts both inner and outer races 106a, 106b at the contact points 207, 208.
However, when the outer race 106b inclines in the counter clockwise direction in FIG. 5, gaps between the inner and outer races 106a and 106b expand at the lower left row and the upper right row. Thus, as shown in FIG. 6, gaps occur between the lower side ball 106c' and the inner and outer races 106, 106b at the left row, and the lower side ball 106c' rotates in accordance with the outer race 106b rotating in an arrow G direction. As a result, the lower side ball 106c' falls from the inner race 106a to the outer race 106b due to the gravity and centrifugal forces, and collides with the outer race 106b, thereby creating unwanted noise.
Additionally, as shown in FIG. 7, there arise gaps between the upper side ball 106c" and the inner and outer races 106a at the right row. Thus, the upper side ball 106c" falls due to gravity and collides with the inner race 16a, also creating unwanted noise.
When the vehicle engine operates at high rotation speed, the centrifugal force increases, so that the balls 106c always contact the outer race 106b. Thus, the noise caused by the collisions between the balls 106c and the races 106a, 106b is not generated. Therefore, the above described noise problem especially exists when the bearing 106 is used at a low rotation speed, or is used with a rotation source operating from a low rotation speed through a high rotation speed, such as on a vehicle engine.